Leading scorer Gavin Tomlin bagged a fine double against his former employers and midfielder Kevan Hurst also netted as the visitors stretched their impressive unbeaten run to 14 matches in all competitions.
Paul Sturrock's side were always in control at the London Borough of Barking and Dagenham Stadium and needed just 59 seconds to take the lead.
And it was former Dagenham striker Tomlin who struck from close range, albeit a deflected goal.
Luke Howell rifled wide at the other end as the hosts went close to levelling, before Tomlin bagged his second to double the visitors' lead after 14 minutes. Again it was a close-range finish as Tomlin made it 13 goals in his last 10 matches.
Sam Williams had a shot saved by Southend 'keeper Paul Smith and also headed wide of the upright, but the away side dominated and Hurst put the contest out of reach when he found the top left corner with a superb strike 10 minutes after the break.
Williams headed against the roof of the bar from a Femi Ilesanmi cross with 11 minutes left, but by then the damage had been done.